1. Field of the Invention
The present invention relates to a central processing unit and a driving method thereof.
2. Description of the Related Art
Central processing units (CPUs) have a variety of architectures depending on their usage, and an architecture called a stored-program method is a predominant architecture of CPUs. In a stored-program CPU, an instruction and data needed for carrying out the instruction are stored in a semiconductor memory unit (hereinafter simply referred to as a memory unit), and the instruction and the data are sequentially read from the memory unit, whereby the instruction is carried out.
The memory unit includes, besides a main memory unit for storing data and instructions, a cache memory which can perform data writing and data reading at high speed. In order to reduce access to the low-speed main memory unit and speed up the arithmetic processing, a cache memory is provided in an information processing unit to be located between an arithmetic unit (also referred to as an operation part) or a control unit (also referred to as a control part) of the information processing unit and the main memory unit. In general, a static random access memory (SRAM) or the like is used as a cache memory. For example, Patent Document 1 discloses a structure in which a volatile memory such as an SRAM and a nonvolatile memory are used in combination as a cache memory.
The capacitance of a cache memory provided in a central processing unit increases year after year. Accordingly, the proportion of power consumption of a cache memory to the total consumption of a central processing unit remarkably increases; thus, various methods have been suggested in order to reduce power consumption of the cache memory.
For example, a method in which a cache memory is divided into several blocks and the less frequently used blocks (also referred to as lines) acquired by historical information are operated with a low threshold voltage has been suggested. Further, a method for stopping power supply to a cache line which is less likely to be accessed has also been suggested.